The present invention relates generally to gain control in communications systems, and specifically to gain control of repeaters within communications systems.
A cellular communications network which operates in regions closed off from electromagnetic radiation, such as within buildings or inside tunnels, typically achieves coverage within the closed-off regions by using a repeater system. The repeater system comprises a first repeater outside the region communicating directly with a base-station transceiver system (BTS), a second repeater inside the region communicating directly with mobile units within the closed-off region, and cabling connecting the two repeaters. It is known in the art that varying signal levels at the BTS or at the mobile units adversely affect operation of the network, by effectively increasing the noise in the network, thus decreasing signal/noise levels. The effect is overcome by constantly monitoring signal levels at the BTS and at the mobile units, and most preferably adjusting gains of the mobile units to maintain the signal levels as constant as possible. With the interposition of a repeater system between the BTS and the mobile units, it is important that signal level changes caused by changes of gain within the repeater system are minimized, and that they are made slowly, to ensure that the repeater system remains substantially transparent to the network.
U.S. Pat. No. 5,799,005 to Soliman, whose disclosure is incorporated herein by reference, describes a system and method for estimating the quality and path loss associated with a communications channel. The estimate is made by measuring the power of a pilot signal received by a communications receiver. The communications receiver measures a received signal power, and also makes a relative pilot strength measurement of the received pilot signal. The power of the pilot signal is then computed using the received signal power and the relative pilot strength measurement. A base station also transmits an indication of the power at which the pilot signal was transmitted. An estimate of the path loss is then made by determining the difference between the indicated power of the transmitted pilot signal and the received pilot signal power.
U.S. Pat. No. 5,991,284 to Willenegger, et al., whose disclosure is incorporated herein by reference, describes a method for controlling the transmitted power of each subchannel generated by a station transmitting a channel. The station generates a channel made up of a sum of subchannels so that each subchannel or group of subchannels is amplified with a unique gain value that is varied in accordance with subchannel power control messages from a receiving station. The receiving station generates each subchannel power control message after monitoring and calculating metrics based on that received subchannel.
It is an object of some aspects of the present invention to provide a method and apparatus for controlling a gain between repeaters in a cellular communications network.
It is a further object of some aspects of the present invention to provide a method and apparatus for setting gains in an automatic calibration process for a cellular communications network.
In preferred embodiments of the present invention, a base-station transceiver system (BTS) communicates with a master repeater unit within a cellular communications network. The master unit communicates via cables with remote units, which remote units in turn communicate with mobile transceivers which are cut-off from direct communication with the BTS, inside a building, for example. The communication comprises a forward transmission path from the master unit to the remote units, and a reverse transmission path from the remote units to the master unit. The communication between the master unit and remote units enables the mobile transceivers to function within the network.
During an initialization phase, a forward gain and a reverse gain of each of the remote units are set separately, preferably in a substantially automatic manner. The forward gains are set so as to generate default power outputs, preferably substantially equal, at each remote unit. Alternatively, the power outputs of each remote unit are set according to settings transmitted thereto from the master unit. The reverse gain of each remote unit is adjusted in response to forward parameters, such as cable insertion loss, measured during the installation phase, and known differences of these parameters for the reverse transmission path.
In order to set the forward gain, a pilot reference frequency is injected after an input stage of the master unit, most preferably at a level substantially equal to the level generated by the input stage when the latter is operational. The input stage is de-activated during the initialization phase, so that only the pilot signal is transmitted in the system during initialization. The pilot reference signal is most preferably a narrow-band signal at a frequency within a band used for communication within the network. The pilot signal is detected by a respective first detector comprised in each remote unit. Using the known input level and the level read by the first detector a forward gain of the remote unit is then set so as to generate the required power output for the specific unit. Also, a cable forward insertion loss between the master and specific remote unit is calculated from the two levels.
Reverse gain levels for each remote unit are evaluated by extrapolating the forward insertion loss measurements found during the initialization phase, to find a reverse insertion loss. The extrapolation takes account of differences between the forward and reverse paths. The differences comprise cable loss differences caused by differences in transmission frequencies between the forward and reverse paths, as well as different insertion losses of elements in the two paths. Most preferably, the reverse gain set for each remote unit is generally greater than the cable reverse insertion loss by a predetermined value, such as 5 dB.
Each remote unit comprises an output stage which is de-activated during a period when the specific remote unit is being initialized. The first detector of the remote unit is positioned before the output stage. The de-activated master unit input stage and remote unit output stages act as isolators. Thus, forward and reverse gain adjustments may be implemented for each remote unit without the pilot signal generating any external interference, and without external signals causing interference with the adjustments.
In an operational phase the pilot is de-activated, and the master unit input stage and the output stage of each remote unit are activated. A forward gain of the master unit input stage is set to generate a substantially fixed nominal output level. (Most preferably, the level at which the pilot is injected in the initialization phase substantially equals this output level.) Most preferably, a master unit reverse gain is set so that a system reverse gain is substantially equal to a system forward gain. Alternatively, the reverse gain is set to be different from the forward gain by a predetermined value.
Each remote unit comprises a second detector which is used to monitor power output from the output stage of the respective remote unit. Forward and reverse gains of the master unit and each of the remote units are maintained as constant as possible in order to maintain system gain settings substantially unchanged from their installation settings, which in turn maintains signal/noise ratios in the forward and reverse transmission paths. During operation of the system, forward and reverse gains and power outputs of each of the remote units are monitored and adjusted when necessary, for example when system parameters change, so as to maintain the forward and reverse gains substantially unchanged, according to the site design. The input master unit stage and/or one or more of the remote unit output stages may be temporarily de-activated during the operational phase in order to perform measurements, such as updating of power loss values, normally implemented during the installation phase.
Incorporating stages in the master and remote units which can be de-activated so as to act as isolators, and which can be activated to operate within the units, leads to a highly flexible system for maintaining gains of the units at optimal levels, with substantially no incoming or outgoing interference.
There is therefore provided, according to a preferred embodiment of the present invention, a method for adjusting a radio-frequency (RF) power level in a cellular communications network, the network including a first plurality of signal-transmission-elements adapted to receive communication signals as a first variable-gain repeater coupled to a second plurality of signal-transmission-elements adapted to transmit the communication signals as a second variable-gain repeater, the method including:
de-activating at least some of the first plurality of signal-transmission-elements so that the communication signals are not received in the first plurality;
de-activating at least some of the second plurality of signal-transmission-elements so that the communication signals are not transmitted from the second plurality;
injecting a reference signal having a predetermined injected power level at the first plurality of signal-transmission-elements;
receiving the reference signal at the second plurality of signal-transmission-elements;
measuring a received power level of the reference signal at the second plurality of signal-transmission-elements;
comparing the injected and received power levels; and
responsive to the comparison, setting a gain of at least one of the signal-transmission-elements included in the first and second pluralities.
Preferably, the method includes:
activating the at least some of the first plurality of signal-transmission-elements;
receiving at the first plurality of signal-transmission-elements the communication signals from a first region;
activating the at least some of the second plurality of signal-transmission-elements; and
transmitting the communication signals to a second region from the second plurality of signal-transmission-elements.
Preferably, the first region and the second region do not overlap.
Preferably, injecting the reference signal includes measuring the predetermined injected power level with a first detector, measuring the received power level includes measuring the received power level with a second detector, and activating the at least some of the second plurality of signal-transmission-elements includes activating a power amplifier and setting a power level output of the power amplifier responsive to a power-amplifier output measured by a third detector and the predetermined injected power level measured by the first detector and the received power level measured by the second detector.
Further preferably, the method includes setting an alarm responsive to the power-amplifier level and the received power level.
Preferably, the method includes:
receiving the communication signals at a frequency outside a predetermined forward-intermediate-frequency (FWD-IF) band at the first plurality of signal-transmission-elements;
mixing the communication signals with a local oscillator (LO) signal so as to generate a forward IF signal within the FWD-IF band;
conveying the forward IF signal to the second plurality of signal-transmission-elements; and
mixing the forward IF signal with the LO signal so as to recover information in the communication signals at the second plurality of signal-transmission-elements.
Further preferably, injecting the reference signal includes generating the reference signal at a reference frequency within the FWD-IF band.
Preferably, the method includes:
receiving the communication signals at a frequency outside a predetermined reverse-intermediate-frequency (REV-IF) band, different from the FWD-IF band, at the second plurality of signal-transmission-elements;
mixing the communication signals with the LO signal so as to generate a reverse IF signal within the REV-IF band;
conveying the reverse IF signal to the first plurality of signal-transmission-elements; and
mixing the reverse IF signal with the LO signal so as to recover information in the communication signals at the first plurality of signal-transmission-elements;
and setting the gain of the one of the first plurality of signal-transmission-elements and the gain of the one of the second plurality of signal-transmission-elements includes:
determining gain values at the reference frequency; and
extrapolating the gain values to the REV-IF band responsive to parameters of the first and second pluralities of signal-transmission-elements and of a cable coupling the pluralities.
Preferably, the first plurality of signal-transmission-elements and the second plurality of signal-transmission-elements are coupled by a cable, and the method includes determining an effective length and an effective loss of the cable responsive to the comparison.
Further preferably, the effective loss of the cable includes a forward effective loss responsive to a forward-intermediate-frequency and a reverse effective loss responsive to a reverse-intermediate-frequency different from the forward-intermediate-frequency.
Preferably, the method includes:
broadcasting an expected-output-level value from the first plurality of signal-transmission-elements;
receiving the expected-output-level value at the second plurality of signal transmission-elements; and
setting the gain of the at least one of the signal-transmission-elements included in the second plurality responsive to the expected-output-level value.
Preferably, setting the gain includes setting a forward-gain for the communication signals in a forward path included in the first and second pluralities, and setting a reverse-gain for the communication signals in a reverse path included in the first and second pluralities.
Preferably, the forward-gain and the reverse-gain differ by a pre-determined value.
Preferably, the first plurality of signal-transmission-elements receive the communication signals at a signal-reception-level, and injecting the reference signal includes injecting the reference signal at an injection-level substantially the same as the signal-reception-level.
There is further provided, according to a preferred embodiment of the present invention, apparatus for adjusting a radio-frequency (RF) power level in a cellular communications network, including:
a first plurality of signal-transmission-elements coupled as a first variable-gain repeater;
a second plurality of signal-transmission-elements coupled as a second variable-gain repeater;
a coupling connecting the first and second pluralities of signal-transmission-elements, which is adapted to convey communication signals therebetween;
a reference oscillator, which is adapted to inject a reference signal having a predetermined injected power level at the first plurality of signal-transmission-elements;
a receiver, which is adapted to measure a received power level of the reference signal at the second plurality of signal-transmission-elements;
switching circuitry, which is adapted to de-activate at least some of the first and second pluralities of signal-transmission-elements so that the reference signal is not radiated from the coupling and the first and second pluralities of signal-transmission-elements; and
control circuitry which is adapted, responsive to the injected power level and the received power level, to set a gain of one of the signal-transmission-elements included in the first and second pluralities.
Preferably, the switching circuitry is adapted to activate the at least some of the first plurality of signal-transmission-elements and the at least some of the second plurality of signal-transmission-elements so that the first plurality of signal-transmission-elements receives the communication signals from a first region and the second plurality of signal-transmission-elements transmits the communication signals to a second region.
Preferably, the first region and the second region do not overlap.
Preferably, the second plurality of signal-transmission-elements includes a power amplifier, and the apparatus includes:
a first detector which is adapted to measure the predetermined injected power level;
a second detector which is adapted to measure the received power level; and
a third detector which is adapted to measure a power-amplifier-level output of the power amplifier, so that a power-amplifier level is set responsive to the power-amplifier-level output and the predetermined injected power level and the received power level.
Preferably, the apparatus includes an alarm which is activated responsive to the power-amplifier level and the received power level.
Preferably, the first plurality of signal-transmission-elements are adapted to receive the communication signals at a frequency outside a predetermined forward-intermediate-frequency (FWD-IF) band and to mix the communication signals with a local oscillator (LO) signal so as to generate a forward IF signal within the FWD-IF band; and
the second plurality of signal-transmission-elements are adapted to receive the forward IF signal and to mix the forward IF signal with the LO signal so as to recover information in the communication signals.
Further preferably, the reference oscillator is adapted to generate the reference signal at a reference frequency within the FWD-IF band.
Preferably, the second plurality of signal-transmission-elements is adapted to receive the communication signals at a frequency outside a predetermined reverse-intermediate-frequency (REV-IF) band, different from the FWD-IF band, and to mix the communication signals with the LO signal so as to generate a reverse IF signal within the REV-IF band;
the first plurality of signal-transmission-elements is adapted to mix the reverse IF signal with the LO signal so as to recover information in the communication signals; and
the control circuitry is adapted to determine gain values at the reference frequency and to extrapolate the gain values to the REV-IF band responsive to parameters of the first and second pluralities of signal-transmission-elements and of a cable coupling the pluralities.
Preferably, the coupling includes a cable, and the control circuitry is adapted to determine an effective length and an effective loss of the cable responsive to the injected power level and the received power level.
Further preferably, the effective loss of the cable includes a forward effective loss responsive to a forward-intermediate-frequency and a reverse effective loss responsive to a reverse-intermediate-frequency different from the forward-intermediate-frequency.
Preferably, the first plurality of signal-transmission-elements are adapted to broadcast an expected-output-level value therefrom;
the second plurality of signal-transmission-elements are adapted to receive the expected-output-level value; and
the control circuitry is adapted to set the gain of the at least one of the signal-transmission-elements comprised in the second plurality responsive to the expected-output-level value.
Preferably, the first and second pluralities include:
forward-path-elements for the communication signals following a forward path from the first to the second plurality; and
reverse-path-elements for the communication signals following a reverse path from the second to the first plurality;
and the control circuitry is adapted to set a forward-gain for the forward-path-elements and a reverse-gain for the reverse-path-elements.
Preferably, the forward-gain and the reverse-gain differ by a pre-determined value.
Preferably, the first plurality of signal-transmission-elements receive the communication signals at a signal-reception-level, and the reference oscillator is adapted to inject the reference signal at an injection-level substantially the same as the signal-reception-level.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: